Methods for creating a phrase thesaurus

ABSTRACT

The invention enables creation of grammar networks that can regulate, control, and define the content and scope of human-machine interaction in natural language voice user interfaces (NLVUI). More specifically, the invention concerns a phrase-based modeling of generic structures of verbal interaction and use of these models for the purpose of automating part of the design of such grammar networks.

This application is a divisional of U.S. application Ser. No.10/096,194, filed Mar. 11, 2002, which is a continuation of U.S.application Ser. No. 09/580,059, filed May 27, 2000, which claims thebenefit of U.S. Application Ser. No. 60/136,970, filed May 28, 1999, allof which are incorporated here by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the creation of grammar networks thatregulate, control, and define the content and scope of human-machineinteraction in natural language voice user interfaces (NLVUI). Morespecifically, the invention concerns a phrase-based modeling of genericstructures of verbal interaction and use of these models for the purposeof automating part of the design of such grammar networks.

2. Related Art

In recent years, a number of routine over-the-phone transactions such asvoice dialing and collect call handling, as well as some commercial callcenter self-service applications, have been successfully automated withspeech recognition technology. Such systems allow users to access e.g.,a banking application or ticket reservation system, remotely, and toretrieve information or complete simple transactions by using voicecommands.

a. Limitations and Unsolved Problems in Current Technology

Current technology limits the design of spoken dialogue systems in termsof both complexity and portability. Systems must be designed for aclearly defined task domain, and users are expected to respond to systemprompts with short, fixed voice commands. Systems typically work well aslong as vocabularies remain relatively small (200-500 words), choices atany point in the interaction remain limited and users interact with thesystem in a constrained, disciplined manner.

There are two major technological barriers that need to be overcome inorder to create systems that allow for more spontaneous userinteraction: (1) systems must be able to handle more complex tasks, and(2) the speech interface must become more “natural” if systems areexpected to perform sophisticated functions based on unrestrained,natural speech or language input.

A major bottleneck is the complexity of the grammar network that enablesthe system to recognize natural language voice requests, interpret theirmeaning correctly, and respond appropriately. As indicated above, thisnetwork must anticipate, and thus explicitly spell out, the entirevirtual space of possible user requests and/or responses to any givensystem prompt. To keep choices limited, the underlying recognitiongrammars typically process requests in a strictly predetermined,menu-driven order.

Another problem is portability. Current systems must be task specific,that is, they must be designed for a particular domain. An automatedbanking application cannot process requests about the weather, and,conversely, a system designed to provide weather information cannotcomplete banking transactions. Because recognition grammars are designedby hand and model domain specific rather than generic machine-humaninteraction, they cannot be easily modified or ported to another domain.Reusability is limited to certain routines that may be used in more thanone system. Such routines consist of sub-grammars for yes-no questionsor personal user data collection required in many commercialtransactions (e.g., for collecting name, addresses, credit cardinformation etc.). Usually, designing a system in a new domain meansstarting entirely from scratch.

Even though the need for generic dialogue models is widely recognizedand a number of systems claim to be portable, no effective andcommercially feasible technology for modeling generic aspects ofconversational dialogue currently exists.

b. Current System Design and Implementation

The generated dialogue flow and the grammar network can be dauntinglycomplex for longer interactions. The reason is that users always manageto come up with new and unexpected ways to make even the simplestrequest, and all potential input variants must be anticipated in therecognition grammar. Designing such recognition grammars, usually bytrained linguists, is extremely labor-intensive and costly. It typicallystarts with a designer's guess of what users might say and requireshours of refinement as field data is collected from real usersinteracting with a system simulation or a prototype.

c. Stochastic Versus Rule-Based Approaches to Natural LanguageProcessing

Since its beginnings, speech technology has oscillated betweenrule-governed approaches based on human expert knowledge and those basedon statistical analysis of vast amounts of data. In the realm ofacoustic modeling for speech recognition, probabilistic approaches havefar outperformed models based on expert knowledge. In natural languageprocessing (NLP), on the other hand, the rule-governed, theory-drivenapproach continued to dominate the field throughout the 1970's and1980's.

In recent years, the increasing availability of large electronic textcorpora has led to a revival of quantitative, computational approachesto NLP in certain domains.

One such domain is large vocabulary dictation. Because dictation coversa much larger domain than interactive voice-command systems (typically a30,000 to 50,000 word vocabulary) and does not require an interpretationof the input, these systems deploy a language model rather than arecognition grammar to constrain the recognition hypotheses generated bythe signal analyzer. A language model is computationally derived fromlarge text corpora in the target domain (e.g., news text). N-gramlanguage models contain statistical information about recurrent wordsequences (word pairs, combinations of 3, 4, or n words). They estimatethe likelihood that a given word is followed by another word, thusreducing the level of uncertainty in automatic speech recognition. Forexample, the word sequence “A bear attacked him” will have a higherprobability in Standard English usage than the sequence “A bare attackedhim.”

Another domain where probabilistic models are beginning to be used isautomated part-of-speech analysis. Part-of-speech analysis is necessaryin interactive systems that require interpretation, that is, aconceptual representation of a given natural language input. Traditionalpart-of-speech analysis draws on explicit syntactical rules to parsenatural language input by determining the parts of an utterance and thesyntactic relationships among these parts. For example, the syntacticalrule S->NP VP states that a sentence S consists of a noun phrase NP anda verb phrase VP.

Rule-based parsing methods perform poorly when confronted withsyntactically ambiguous input that allows for more than one possiblesyntactic representation. In such cases, linguistic preferences capturedby probabilistic models have been found to resolve a significant portionof syntactic ambiguity.

Statistical methods have also been applied to modeling larger discourseunits, such as fixed phrases and collocations (words that tend to occurnext to each other, e.g. “eager to please”). Statistical phrase modelinginvolves techniques similar to the ones used in standard n-gram languagemodeling, namely, collecting frequency statistics about word sequencesin large text corpora (n-grams). However, not every n-gram is a validphrase, for example, the sequence “the court went into” is a valid4-gram in language modeling, but only “the court went into recess” is aphrase. A number of different methods have been used to derive validphrases from n-grams, including syntactical filtering, mutualinformation, and entropy. In some cases, statistical modeling of phrasesequences has been found to reduce lexical ambiguity. Others have used aphrase-based statistical modeling technique to generate knowledge basesthat can help lexicographers to determine relevant linguistic usage.

Experiments in training probabilistic models of higher-level discourseunits on conversational corpora have also been shown to significantlyreduce the perplexity of a large-vocabulary continuous speechrecognition task in the domain of spontaneous conversational speech.Others have modeled dialogue flow by using a hand-tagged corpus in whicheach utterance is labeled as an IFT (illocutionary force type).Probabilistic techniques have also been used to build predictive modelsof dialogue structures such as dialogue act sequences. The bottleneck inall of these experiments is the need for hand-tagging both training andtesting corpora.

Another recent application of a probabilistic, phrase-based approach toNLP has been in the field of foreign language pedagogy, where it hasbeen proposed as a new method of teaching foreign languages. MichaelLewis, in his book, Implementing The Lexical Approach (Howe, Engl, 1997)challenges the conventional view that learning a language involves twoseparate cognitive tasks: first, learning the vocabulary of thelanguage, and second, mastering the grammatical rules for combiningwords into sentences. The lexical approach proposes instead thatmastering a language involves knowing how to use and combine phrases inthe right way (which may or may not be grammatical). Phrases, in Lewis'ssense are fixed multi-word'chunks of language, whose likelihood ofco-occurring in natural text is more than random. Mastering a languageis the ability of using these chunks in a manner that produces coherentdiscourse without necessarily being rule-based.

SUMMARY OF THE INVENTION

In one aspect, the present invention concerns modeling generic aspectsof interactive discourse based on statistical modeling of phrases inlarge amounts of conversational text data. It involves automaticallyextracting valid phrases from a given text corpus, and clustering thesephrases into syntactically and/or semantically meaningful equivalentclasses. Various existing statistical and computational techniques arecombined in a new way to accomplish this end. The result is a largethesaurus of fixed word combinations and phrases. To the extent thatthis phrase thesaurus groups similar or semantically equivalent phrasesinto classes along with probabilities of their occurrence, it containsan implicit probabilistic model of generic structures found ininteractive discourse, and thus can be used to model interactions acrossa large variety of different contexts, domains, and languages.

In another form of the present invention, this thesaurus provides a datastructure in which variations of saying the same thing and theirassociated probabilities can be looked up quickly. It forms the keyelement of an application that supports the rapid prototyping of complexrecognition grammars for voice-interactive dialogue systems.

The present invention has a number of significant advantages overexisting techniques for designing voice recognition grammars. Mostsignificantly, it automates the most laborious aspects of recognitiongrammar design, namely, the need to generate, either by anticipation orby empirical sampling, potential variants of responses to any givensystem prompt. Secondly, it eliminates the need for expensive user datacollection and hand coding of recognition grammars. Thirdly, theinvention allows developers without specialized linguistic knowledge todesign much more complex networks than conventional design techniquescan support. In sum, the invention enables a developer to create morecomplex and better performing systems in less time and with fewerresources.

In another aspect of the invention, a compiled thesaurus (containingonly the phrases incorporated into any given recognition grammar) isincorporated into a natural language understanding (NLU) component thatparses the recognizer output at run-time to derive a conceptual meaningrepresentation. Because phrases consist of words in context, they arepotentially less ambiguous than isolated words. Because a phrase-basedparser can draw on the linguistic knowledge stored in a largeprobabilistic phrase thesaurus, it is able to parse utterances muchfaster and with higher accuracy than conventional rule-based parsers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a two-dimensional vector space for the phrases “canyou show me . . . ” and “can you hand me . . . .”

FIG. 2 illustrates a matrix representation of a singular valuedecomposition algorithm.

FIG. 3 illustrates a simplified matrix representation of a singularvalue decomposition algorithm

FIG. 4 is an example of a dialogue flow chart for a simple restaurantinformation request.

FIG. 5 shows a type of network recognition grammar for user responses tothe system prompt: “What kind of food would you like to eat?”.

FIG. 6 illustrates the place of the present invention within anapplication that is controlled by a voice-interactive natural languageuser interface.

DETAILED DESCRIPTION OF THE INVENTION

1. Phrase-Based Dialogue Modeling

The present invention can enable a person with no special linguisticexpertise to design a dialogue flow for an interactive voiceapplication. It can be used to automatically generate a recognitiongrammar from information specified in a dialogue flow design. The keyelement in the present invention is a large, machine-readable databasecontaining phrases and other linguistic and statistical informationabout dialogue structures. This database provides the linguisticknowledge necessary to automatically expand a call-flow design into arecognition grammar. The following is a description of the components ofthe invention, how they are generated and how they work together withinthe overall system.

a. Phrase Thesaurus

The phrase thesaurus is a large database of fixed word combinations inwhich alternative ways of saying the same thing can be looked up. Thephrases are arranged in the order of frequency of occurrence, and theyare grouped in classes that contain similar or semantically equivalentphrases. The following is an example of a class containinginterchangeable ways of confirming that a previous utterance by anotherspeaker has been understood:

-   -   I understand    -   I hear you    -   [I] got [you: your point: it]    -   I see your point    -   I [hear: see: know: understand] [what you're saying: what you        mean]    -   I follow you    -   [I'm: I am] with you [there]    -   I [hear: read] you loud and clear        (Example based on Michael Lewis, Implementing the Lexical        Approach: Putting Theory into Practice, Howe, Engl., 1997.)

The database comprises anywhere from 500,000 and 1 million phraseentries, plus a vocabulary of lexical items containing objects,locations, proper names, dates, times etc. that are used to fill theslots in phrase templates such as “how do I get to . . . ?”. Somepartial phrases may occur in several different groupings. For example,the sub-phrase “I know” in “I know what you mean” may also occur inanother class containing alternate ways of challenging a speaker:

-   -   [I know: I'm sure: I believe] you're [wrong: mistaken]        As a result, some phrase classes may be overlapping or contain        cross-references between partial phrases.        b. Building a Phrase Thesaurus

The phrase thesaurus is generated automatically by a series of computerprograms that operate on large amounts of natural language text data.The programs are executed sequentially and in a fixed order, each takingthe output of the previous program as its input, and processing itfurther. Taken together, the programs take a large text corpus as theirinput, and output a phrase thesaurus of the type described in section a.above. Some of the steps involved in this process are based on standardalgorithms that have been used in various aspects of computationallinguistics to process large machine readable corpora. These algorithmsare used and combined within the present invention in a new way toaccomplish the goal of automatically deriving a phrase thesaurus.

c. Linguistic Assumptions Underlying the Invention

The present invention makes the following linguistic assumptions:

-   1. Language in general, and conversational speech in particular,    consists of phrases rather than of isolated vocabulary items, the    combination of which is governed by grammatical rules.-   2. A phrase is a fixed, multi-word chunk of language of an average    length between 1 and 7 words that conveys a unique idiomatic sense    depending on just that particular combination of words. The words    that make up a phrase may or may not occur next to each other (e.g.,    the phrase “to make sense” can be separated by “a whole lot of,”    “not much,” etc.).-   3. The use of phrases is governed by conventions of usage and    linguistic preferences that are not always explicable with reference    to grammatical rules. The phrase “on the one hand” loses its unique    phrasal sense if “hand” is replaced by “finger.” “On the one finger”    is not a legitimate phrase in Standard English, even though it is    perfectly grammatical. Being able to use just the right phrases    signals native fluency in a speaker.-   4. There are at least tour types of phrases:    -   (classification based on Lewis, 1997 and Smadja, 1994). The        typology is not meant to be exhaustive or complete; other        classifications may be possible.    -   a) Polywords: generally 1-3 word fixed phrases conveying a        unique idiomatic sense. Polywords allow for no variation or        reversal of word order. Example: “by the way,” “nevertheless,”        “bread and butter,” “every now and then.”    -   b) Collocations: words that occur next to each other in more        than random frequencies and in ways that are not generalizable:        -   Example: “perfectly acceptable,” “stock market slide,”            “sales representative.”    -   Variation in collocations is possible, but restricted by        linguistic usage: “a tall building,” “a tall boy” (but not: “a        high building,” “a high boy”); “to take a look at a problem”        (not: “to gaze at a problem”); “anxiety attack” (not “fear        attack”), but also an “asthma attack,” a “hay-fever attack.”    -   (c) Standardized, idiomatic expressions with limited        variability, often used in formulaic greetings and social        interaction routines:        -   Example: “How's it going?” “How are you doing?” a “Thanks,            I'm fine [great: terrific].” “Talk to you later.”    -   (d) Non-contiguous phrases: functional frames containing one or        more slots that can be filled by a limited number of words. The        meaning of the phrase is determined by the filler word. The set        of legitimate filler words tends to be determined by world        knowledge rather than linguistic usage. Example: “Can you pass        me the . . . , please?” Here, the filler can be any small object        that can be “passed on” by hand: “salt,” “pepper,” “bread,”        “water,” but not “house,” “tree,” “sewing-machine” etc.        -   I have a . . . in my shoe can be filled by e.g., “stone,”            “pebble,” “something”, but not by “elephant.”-   5. Because they are fixed in the mental lexicon of the speakers of    the language, some word combinations are more likely to be    observed/chosen in actual discourse than other combinations. This is    why usage patterns and their frequencies can be analyzed using    statistical methods, and can be captured in probabilistic models    that reveal these patterns.-   6. Phrases are relatively unambiguous in their meaning or intention.    Ambiguity arises when an utterance can have more than one conceptual    meaning. The source of ambiguity can be either lexical (a word can    have 2 or more unrelated meanings. E.g., “suit”=1. a piece of    clothing, 2. a legal dispute), syntactic (a sentence can have two or    more different and equally plausible parses (e.g. “he killed the man    with a knife,” where the modifier “with a knife” can either refer to    VP (the act of killing) or to the NP (the object of killing).    Because phrases use words in context, they reduce semantic ambiguity    (wearing a suit vs. filing a suit) and some cases of syntactic    ambiguity.-   7. Phrasal usage is not an exclusive property of spoken,    conversational language. Rather, phrase usage pertains to all forms    and genres of spoken and written discourse. However, each of these    genres may use different types of phrases, and a computational    analysis of linguistic preferences in terms of phrase frequencies    and probabilities is likely to reveal different patterns of usage    depending on the genre.-   8. Nor is phrasal usage an exclusive property of English. Most    languages are governed by it albeit in different ways. Generally    speaking, phrases do not translate word for word into other    languages. A literal translation, for example, of “get your act    together” into German yields a meaningless construct “bring deine    Tat zusammen.” However, many phrases have functional phrase    equivalents in other languages, e.g., “getting one's act    together”=>“sich zusammenreiBen.”    d. Goal of the Invention

The goal of the present invention is twofold:

-   -   1. To implement a phrase-based, corpus driven natural language        processing technique that can reveal overarching discourse        patterns without requiring laborious hand-tagging of training        data in terms of syntactic, semantic, or pragmatic utterance        features. As Lewis puts it: “Grammar tends to become lexis as        the event becomes more probable” (p. 41). That is to say,        syntactic, semantic, and pragmatic structures are embedded in        the phrase and are modeled along with it, provided the analysis        is based on a conversational speech corpus large enough for        statistical modeling.    -   2. To implement the process described under 1) above in such a        way that the resulting linguistic knowledge can be stored in a        machine readable database, and used (and reused repeatedly) in a        computer system designed to generate recognition grammars for        voice-interactive dialogue systems.        e. Data Resources

Statistical modeling of any kind requires a vast amount of data. Tobuild a sizable phrase thesaurus of 500,000 to 1 Million entriesrequires a large source corpus (on the order of 1 billion words).However, smaller and more specialized corpora may be used to modelphrases in a particular domain. For a phrase thesaurus covering thedomain of interactive discourse, a number of diverse resources may beused to compile a text corpus for language. Such resources include butare not limited to:

-   -   1. Transcribed speech databases for task oriented interactive        discourse (SWITCHBOARD, CallHome, and TRAINS (available from the        Linguistic Data Consortium (LDC) at www.ldc.upenn.edu).    -   2. User data collected from verbal interactions with existing        dialogue systems or with simulations of such systems.    -   3. Closed caption data from television programs containing large        amounts of interactive dialogue, such as talk shows, dramas,        movies, etc. Television transcripts tend to be highly accurate        (95%-100% for off-line captioned programs) (Jensema, 1996). As a        consequence, virtually unlimited amounts of data can be        purchased from places that gather and disseminate this data.

Television transcripts are a good way of supplementing databases oftask-oriented discourse (1. and 2.) Even though most television showsare scripted, they nonetheless contain large amounts of common dialogicstructures, good idiomatic English etc. What is missing is mainly thefragmented, discontinuous nature of most conversational speech. However,this difference may well be an advantage in that models based onwell-formed conversational speech might be used to identify and repairelliptical speech.

f. Data Preparation

To prepare the corpus for phrase modeling, it is subjected to anormalization procedure that marks sentence boundaries, identifiesacronyms, and expands abbreviations, dates, times, and monetary amountsinto full words. This normalization process is necessary because thephrase thesaurus is used to create grammars for recognition systems, andrecognizers transcribe utterances as they are spoken, not as they arewritten. This means that monetary amounts, e.g., $2.50, must be spelledout in the recognition grammar as “two dollars and fifty cents” in orderto be recognized correctly. The procedure also eliminatesnon-alphanumeric characters and other errors that are often found intelevision transcripts as a result of transmission errors in the captiondelivery.

The normalization process is carried out by running a sequence ofcomputer programs that act as filters. In the normalization process, rawtext data is taken as input and a cleaned-up, expanded corpus that issegmented into sentence units is output. Sentence segmentation isespecially important because the subsequent phrase modeling proceduretakes the sentence as the basic unit.

The invention can make use of a version of a text normalization toolkitthat has been made freely available to the speech research community(Copyright 1994, University of Pennsylvania, available through theLinguistic Data Consortium).

g. Compiling a Seed Dictionary of Phrase Candidates

The first step and the precondition for building a phrase thesaurus froma corpus is a creating a seed dictionary of likely phrase candidates.Initially, existing on-line idiomatic dictionaries are searched forbasic phrase candidates that are rigid and not subject to grammatical orlexical variation (section 1. C. 4 a-c). The words and phrases arecompiled into a basic phrase list. Less rigid collocations and phrasaltemplates are subject to considerable lexical and grammaticalvariability, and therefore, empirical text data are needed that containactual instances of their use. To compile an initial seed phrasedictionary, we derive collocations automatically from large corpora onthe basis of simple frequency counts, and then subject the results to apost-processing heuristics to eliminate invalid collocations.

Step 1: Deriving N-Grams

We begin by deriving n-gram statistics from a given corpus C1 usingstandard language modeling techniques. For an overview of suchtechniques, see Frederik Jelinek, Frederick, Statistical Methods forSpeech Recognition, MIT, Cambridge Mass., 1997). The procedure generatesinformation about how often word strings of n-word length occur in agiven corpus.

Input: A given Corpus C1->Output: n-gram frequency counts.

We choose n-grams of varying lengths (approximately 1<=n<=7.) N-gramsare sorted in the order of the frequency of their occurrence.

Step 2: Filtering: Deriving Valid Phrase Candidates from N-Grams

The list of n-grams is very large and contains many invalid andmeaningless collocations, phrase fragments, and redundant wordcombinations that are subsumed by larger n-grams.

Take for example, the following sentence: <s> e-mail is replacing to alarge extent direct communication between people </s>.

For 1<=n<=7, n-gram frequency counts on this sentence, includingsentence boundary markers, will return 70 unique n-grams (13 unigrams,12 bigrams, 11 trigrams, 10 4-grams, 9 5-grams, 8 6-grams, and 77-grams). By contrast, the sentence contains only four potentially validphrase candidates, two of which are partially overlapping:

(a) Phrase template: “replacing [ . . . ] communication”

(b) Multi-word: “to a large extent”

(c) Compound noun collocation: “direct communication”

(d) Mixed collocation: “communications between people”

The next step consists of filtering n-grams to eliminate invalid orredundant collocations by implementing a series of computationalmeasures to determine the strength of any given collocation. The problemof n-gram filtering can be approached in a number of different ways, andthe following description is meant to be exemplifying rather than beingexhaustive. Since the goal at this point is to compile a preliminaryseed dictionary of phrases, any of the methods described below can beused, either by themselves or in combination, to identify initial phrasecandidates.

A Frequency-Based Pre-Filtering Method

The simplest filtering method is frequency-based. Computed over a largecorpus, n-grams with high frequency counts are more likely to containstrong collocations than n-grams that occur only once or twice. Weeliminate n-grams below a specific frequency threshold. The threshold islower for large word strings because recurring combinations of largen-grams are rarer, and more likely to contain significant phrasecandidates than shorter strings.

Perplexity/Entropy

Perplexity is a measure for determining the average branching factor ofa recognition network and it is most often used as a measure forevaluating language models. It indicates the probability, computed overan entire network, that any given element can be followed by any other.For example, in a digit recognition system composed of 0-9 digits andtwo pronunciations for 0 (“oh” and “zero”), the perplexity of therecognition grammar exactly equals the number of elements, 11, becausethere are no constraining factors that favor certain digit sequencesover others. Because word sequences underlie various kinds ofconstraints (imposed by syntax, morphology, idiomatic usage etc.)perplexity has been found useful in natural language processing tomeasure the strength of certain collocations (see, for example,Shimohata, S, T. Sugio, J. Nagata, “Retrieving Collocations byCo-occurrence and Word Order Constraints,” Proceedings of the 35thAnnual Meeting of the Association for Computational Linguistics, 1997,pp. 476-481.)

We take each unique n-gram and its associated frequency f(n-gram) andlook at the probability of each word w_(i) that can follow the n-gram.We calculate this probability p(w_(i)) by dividing the frequency inwhich a given word follows the n-gram by the frequency count for then-gram itself

${p\left( w_{i} \right)} = \frac{f({wi})}{f\left( {n - {gram}} \right)}$

If the n-gram is part of a larger, strong collocation, the choice ofwords adjacent to the phrase boundary will be very small, because of theinternal constraint of the collocation. Conversely, the likelihood thata particular word will follow is very high. For example, the wordfollowing the trigram “to a large” will almost always be “extent,” whichmeans, the perplexity is low, and the trigram is subsumed under thefixed collocation “to a large extent.” On the other hand, a large numberof different words can precede or follow the phrase “to a large extent,”and the probability that any particular word will follow is very small(close to 0).

We use a standard entropy measure to calculate the internalco-locational constraints of the n-gram at a given junction wi as:

${H\left( {n - {gram}} \right)} = {\sum\limits_{\lbrack{i = {wordj}}\rbrack}\;{{- {p\left( w_{i} \right)}}\ln\;{p\left( w_{i} \right)}}}$

The perplexity of the n-gram can then be defined as:Prep(n-gram)=e ^(H(n-gram))

We eliminate n-grams with low surrounding perplexity as redundant(subsumed in larger collocations) and keep the ones with perplexityabove a specified threshold t.

Step 3: Deriving Non-Contiguous Phrases

The frequency and perplexity measures described above give us a goodfirst cut at phrase candidates, generating mainly rigid wordcombinations such as compound nouns (“Grade Point Average”), idiomaticexpressions (“How's it going?”) and polywords (“sooner or later”). Thenext objective is to expand the initial seed phrase dictionary byderiving non-contiguous collocations (collocations that are less rigidand contain one or more filler words or phrases, e.g. “Give me . . .please”). There are at least three types of non-contiguous phrases.Assuming that w is any word and p is any phrase, these types can bedistinguished as follows:

Type 1: p₁ . . . p₂

-   -   Two phrases occurring next to each other with more than random        frequency, separated by one or more words that are not        themselves phrases.        -   Example: “refer to [the appendix: the manual: page 220 . . .            ] for more information”

Type 2: p₁ . . . w₁

-   -   A phrase is followed or preceded by one or more filler words,        which are followed or preceded by another word that, together        with the initial phrase, forms a phrase template.        -   Example: “Could you hand me [the salt: your ID . . . ]            please?”

Type 3: w₂ . . . w₂

-   -   A word is followed by one or more filler words, which are        followed by another word that together with the initial word        forms a phrase template.        -   Example: “taking [initial: the first: important . . . ]            steps”

To extract phrases of the types 1 and 2, we first create a list ofcontexts for each phrase. We take each of the phrase candidates obtainedin the first processing phase and retrieve all sentences containing thephrase. We then look at surrounding words in order to identify possibleregularities and co-occurrence patterns with words or phrases notcaptured in the initial n-gram modeling and filtering stage. This can bedone using any of the following methods: frequency counts, normalizedfrequency methods, perplexity, or normalized perplexity.

In order to handle Type 3, we compile a list of the top n most frequentword bigrams separated by up to 5 words. As in the first extractionstage, not every collocation is significant. Again, there are severalways to eliminate invalid collocations that can be used by themselves orin various combinations. Again, this can be done using any of thefollowing methods: frequency counts, normalized frequency methods,perplexity, or normalized perplexity.

Mutual Information

Mutual information is a standard information theoretical measure thatcomputes the strength of a relationship between two points by comparingthe joint probability of observing the two points together with theprobability of observing them independently. In natural languageprocessing, it has been used to establish the strength of an associationbetween words, for example, for use in lexicography (see Kenneth Church,W. & Patrick Hanks, “Word Association Norms, Mutual Information, andLexicography,” Computational Linguistics, 16 (1), 1990: 22-29.)

Given two phrases, q1 and q2 with probabilities p(q1) and p(q2) then themutual information I (q1, q2) is defined as:

${I\left( {{q\; 1},{q\; 2}} \right)} = \frac{p\left( {q_{1},q_{2}} \right)}{{p\left( q_{1} \right)}{p\left( q_{2} \right)}}$

Joint probability can serve as a measure to determine the strength of acollocation within a given window (in our case, a sentence), even if thecollocation is interrupted, as in the case of non-contiguous phrases. Ifthere is a genuine association between two words or word strings, theirjoint probability will be larger than the probability of observing themindependently, so the mutual information I(w1,w2) must be greater than1.

We take our corpus of non-contiguous phrase candidates and compute themutual information for each phrase and the most frequent words or wordsequences surrounding these phrases. We extract the phrase-word orphrase-phrase combinations with the highest joint probability

However, the above formula may generate misleading results in case ofvery frequently used words such as “the,” “it,” or “very good.” In thiscase we will use a slightly modified mutual information defined as:

${I_{new}\left( {{q\; 1},{q\; 2}} \right)} = \frac{p\left( {q_{1},q_{2}} \right)}{p\left( q_{1} \right)}$where q2 is the frequent word or phrase.Probability Distribution

Yet another way to eliminate invalid phrase candidates is to look atprobability distribution of components within each non-contiguous phrasecandidate. For each phrase candidate, we determine a main component anda sub-component (the longer or the more frequent phrases can usually beconsidered as the main component), and then look at the probabilitydistribution of the sub-component with respect to other words orcandidate phrases that co-occur in the same context (i.e., sentence orclause). This algorithm can be formally described as:

$M_{{main},{sub}} = \frac{{f\left( {q_{main},q_{sub}} \right)} - {{Exp}\left( q_{main} \right)}}{{Dev}\left( q_{main} \right)}$where f(q_(main), q_(sub)) is the frequency of the co-occurrence of themain component with the sub-component and Exp (q_(main)) & Dev(q_(main))are the Expected Value and the Standard Deviation of the frequencyoccurrence of q_(main) with all of the sub-components q_(sub),We can assume that if M_(main,sub) is greater than a certain threshold,then the collocation is a valid phrase, otherwise it is not.Hand Checking

A final way of eliminating invalid phrases—especially cases determinedas borderline by the other algorithms—is by having a trained linguist gothrough the resulting phrase dictionary and eliminate the unlikelyphrases.

Step 4: Phrase-Based Corpus Segmentation

As explained in the previous section, a number of measures can be (andhave been) used to automatically derive an initial seed dictionary ofphrase candidates from large corpora. Because all of these methods actmore or less as filters, they can be used in various combinations toextract multi-word phrases and collocations. However, whatever method weuse, the list of derived phrases still contain a large number ofoverlapping phrase candidates, because multiple parses of the samesentence remain a possibility. For example, for the sentence “E-mail isreplacing direct communications between people,” the followingalternative parses are conceivable:

-   -   Parse 1: <s> [E-mail] [is replacing] [direct communications]        [between people] </s>    -   Parse 2: <s> [E-mail] [is replacing direct communications]        [between people] </s>    -   Parse 3: <s> [E-mail] [is replacing] [direct] [communications        between people.] </s>

The problem is similar to the one we encounter when segmenting text forbuilding dictionaries in Chinese or Japanese. In these languages, theconcept of a “word” is less well defined than it is in Europeanlanguages. Each Chinese word is made up of anywhere between one andseven characters, and in Chinese writing, word boundaries are notseparated by white spaces. The problem is augmented by the fact thatcomplete Chinese dictionaries are extremely hard to find, especiallywhen it comes to proper names.

The absence of word boundaries in Chinese or Japanese createssignificant difficulties when building probabilistic language models forlarge vocabulary dictation systems. Word-based n-gram language modelingrequires correct parsing of sentences to identify word boundaries andsubsequently calculate n-gram probabilities. Parsing errors are a commonproblem in Chinese language processing. For example, we may encounter acharacter sequence ABCDE where A, AB, CDE, BCD, D, and E are alllegitimate words in the dictionary. One can quickly note that there aretwo possible parses for this character sequence: [A] [BCD] [E] and [AB][CDE]. Linguists have applied various lexical, statistical, andheuristical approaches, by themselves and in combination, to parseChinese text. Most of these methods can be applied to phrase parsing inEnglish. We describe one statistical, n-gram-based parsing algorithmthat we found particularly efficient and useful. However, other methodscan be used for phrase parsing as well.

The general idea is to implement an N-gram phrase-based language model(a language model that uses phrases rather than single words as thebasis for n-gram modeling), in order to calculate the best parse of asentence. Note that some words may act as phrases as can be seen inSentence 3 (e.g. the word “direct” in the above example). Assuming thelog probability bigram statistics for the example above to be asfollows:

[<s>], [Email]—−5.8

[Email], [is replacing]—−2.4

[Email], [is replacing direct communications]—−6.5

[is replacing], [direct]—−4.7

[is replacing], [direct communications]—−5.4

[direct], [communication between people]—−4.2

[direct communications], [between people]—−6.2

[is replacing direct communications], [between people]—−8.9

[between people] [<s>]—−4.8

[communication between people] [<s>]—−5.9

Given these log probabilities, we can calculate the best phrase-basedparse through a sentence by multiplying the probabilities (or summingthe log probabilities) of each of the bigrams for each possible parse:

Parse1_(Total likelihood)=−5.8+−2.4+−5.4+−6.2+−4.8=−24.6

Parse2_(Total likelihood)=−5.8+−6.5+−8.9+−4.8=−26.0

Parse3_(Total likelihood)=−5.8+−2.4+−4.7+−4.2+−5.9=−23.0

We select the parse with the highest overall likelihood as the bestparse (in this case, parse 1).

A first pass at phrase-based n-gram parsing

In order to create a phrase-based parse of a given text corpus C, weneed a phrase-based language model. Building such a language model,however, requires a pre-parsed text or a dictionary of phrases. In orderto get around this problem, we use a bootstrapping technique thatprovides us with an initial parse of the corpus, which will then formthe basis for building an initial language model that is subsequentlyrefined by iterating the procedure. There are two ways to derive apreliminary parse through the corpus:

-   1. We use a Greedy Algorithm that, whenever it encounters a parsing    ambiguity (more than one parse is possible), selects the longest    phrases (e.g., the parse that produces the longest phrase or the    parse that produces the longest first phrase) from the seed    dictionary. In the above example, parse2 would be selected as the    optimal parse.-   2. We pick the parse that minimizes the number of phrases for each    parse. Assuming that neither the phrase “is replacing direct    communications” (because it is not a very common phrase) nor the    word “direct” are in the seed dictionary, parse 1 would be selected.

Applying either one or both of these algorithms will result in aninitial phrase-based parse of our corpus.

Optimizing the Phrase-Based N-Gram Parse

Once we have an initial parse through our corpus, we divide the corpusinto two sub-corpora of equal size, C1 and C2 and use the seeddictionary of phrases (described in section 1 b-d) to build an initiallanguage model for one of the sub-corpora. We then use this languagemodel to generate an improved segmentation of the other sub-corpus C2.Resulting high-frequency bigrams and trigrams are phrase candidates thatcan be added to the dictionary for improved segmentation.

A significant advantage of using a language modeling technique toiteratively refine corpus segmentation is that this technique allows usto identify new phrases and collocations and thereby enlarge our initialphrase dictionary. A language model based corpus segmentation assignsprobabilities not only to phrases contained in the dictionary, but tounseen phrases as well (phrases not included in the dictionary).Recurring unseen phrases encountered in the parses with the highestunigram probability score are likely to be significant fixed phrasesrather than just random word sequences. By keeping track of unseenphrases and selecting recurring phrases with the highest unigramprobabilities, we identify new collocations that can be added to thedictionary.

There are two ways of implementing this procedure. In the first case, westart a unigram language model, and use this model to segment sub-corpusC2. The segmented sub-corpus C2 is subsequently used to build a new,improved unigram language model on the initial sub-corpus C1. We iteratethe procedure until we see little change in the unigram probabilityscores. At this point we switch to a bigram language model (based onphrase pairs) and reiterate the language modeling process until we seevery little change. Then we use a tri-gram model (based on sequences ofthree phrases) and reiterate the procedure again until we see littlechanges in the segmentation statistics and few new, unseen phrases. Atthis point, our dictionary contains a large number of plausible phrasecandidates and we have obtained a fairly good parse through eachutterance.

In the second case, we implement the same iterative language modelingprocedure, using bigram, trigram, or even n-gram models with largerunits, in the very beginning of the process rather than increasinggradually from unigram to trigram models. One or the otherimplementation may prove more effective, depending on the type of sourcematerial and other variables.

h. Automatically Deriving a Phrase Thesaurus from a Seed Dictionary ofPhrases

The core of the proposed technology is a phrase thesaurus, a lexicon offixed phrases and collocations. The thesaurus differs from the seeddictionary of phrases in that it groups phrases that are close incontent and in some sense interchangeable. The grouping is essential forthe use of the phrase database in the context of the proposed invention,namely, to allow for the retrieval of alternative phrase variants thatcan be used to automatically create a grammar network. We use a matrixmanipulation measure to determine the semantic distance between phrasescontained in our phrase dictionary. Once we have a measure ofcloseness/distance between phrases, we can use this information and astandard clustering algorithm (e.g., Group Average AgglomerativeClustering) to derive sets of semantically similar phrases.

Step 1: Measuring Distance Between Phrases

In order to derive a measure for determining semantic distance betweenphrases, we draw on two basic linguistic assumptions:

-   1. The meaning of a word is determined by its use. Mastering a    language is the ability to use the right words in the right    situation.-   2. The degree of similarity between two words can be inferred from    the similarity of the contexts in which a they appear. Two words are    synonymous if they are completely interchangeable in all contexts.    Two words are similar if they share a subset of their mutual    contexts.

We take these assumptions to hold true not only for isolated words, butfor phrases as well. To determine semantic proximity or distance betweenphrases, we look at the surrounding words and phrases that co-occur withany given phrase P across an entire machine-readable corpus C, andmeasure the extent to which these contexts overlap. For example, we willfind that the phrases “can you hand me . . . ” and “can you pass me . .. ” share a large subset of neighboring words: “salt,” “coffee,”“hammer,” “the paper,” “my glasses,” etc. Conversely, we find no overlapin the neighbors of the phrases “can you pass me . . . ” and “can youtell me . . . . ”

To represent and measure semantic and/or syntactic relationships betweenphrases, we model each phrase by its context, and then use similaritiesbetween contexts to measure the similarity between phrases. One canimagine that each phrase is modeled by a vector in a multi-dimensionalspace where each dimension is used for one context. The degree ofoverlap between vectors indicates the degree of similarity betweenphrases. A simple example illustrates how to represent contextualrelationships between phrases and their associated neighbors in such aspace. For the two phrases, P1: “can you hand me . . . ” and P2: “canyou show me . . . ,” we create an entry in a 2 dimensional matrix foreach time they co-occur with one of two right neighbors, “the salt,” and“your ID.” The example shows that the phrases P1 and P2 share some butnot all of the same contexts. P1 occurs 136 times with “your ID” butnever (0 times) with “the salt,” P2 co-occurs 348 times with “the salt”and 250 times with your ID.

We can capture this co-occurrence pattern geometrically in atwo-dimensional space in which the phrases P1 and P2 represent the twodimensions, and the contexts “the salt” and “Your ID” represent pointsin this space. (FIG. 1) The context the salt is located at point 0,348in this space because it occurs never (0 times) times with P1 and 348times with P2.

The degree of similarity between contexts can be determined by usingsome kind of association measure between the word vectors. Associationcoefficients are commonly used in the area of Information Retrieval, andinclude, among others, the following: Dice coefficient, Jaccard'scoefficient, Overlap coefficient and Cosine coefficient (for anOverview, see C. J. van Rijsbergen, Information Retrieval, 2nd ed.,London, Butterworths, 1979). There is little difference between thesemeasures in terms of efficiency, and several of these coefficients maybe used to determine the difference between phrases. The moststraightforward one is the Cosine coefficient, which defines the angle Θbetween the two word vectors as follows:

${\cos\mspace{11mu}\Theta} = \frac{A^{T}B}{{A}\mspace{11mu}{B}}$Step 2: Singular Value Decomposition

Using either of the formulas described in Step 1 will give us an initialdistance measure between phrases. Assuming the phrase dictionary derivedso far contains N phrases (with N being anywhere from 500,000 to1,000,000), and assuming further that we parameterize each key-phrasewith only the most frequent M phrases (with M being between 500,000 and100,000 depending on a number of variables), then we still have twoproblems:

-   1. The resulting M×N matrix may be too large (500,000×100,000) to    compare vectors.-   2. Because of the sparseness of data, many context phrases or words    will not appear in the context of their respective key phrases. For    less frequent phrases or context phrases, the vector model might    therefore yield misleading and inaccurate results.

In order to get around both of these problems we can use Singular ValueDecomposition (SVD) to reduce the original matrix to a smaller andinformationally richer matrix. We describe the original matrix asfollows: each row is used for one key-phrase and each column is used forone of the M context-phrases. So c_(ij) is the number of occurrences ofthe phrase p_(j) in the context of phrase p_(i). The standard SVDalgorithm for a matrix A of size M×N allows us to express A as a productof a M×N column-orthogonal matrix U, a diagonal matrix S of size N×Nwhose elements are either positive or zero, and transpose of another N×Nrow-orthonormal matrix V. This can be summarized as follows:A=U•S•V ^(T)

The shapes of these matrices can be visualized as a series of columns,as shown in FIG. 2.

The advantage of using SVD is that it allows us to break down the matrixinto its individual components and to reduce the size of the matrix byas much as one order of magnitude by eliminating unwanted or meaninglesscomponents. If the matrix is singular, some of the s_(n) will be zeroand some are going to be very small. By eliminating these elements andreducing the matrix in size, we can make the matrix smaller and moremanageable. Moreover, the reduced matrix A_(new) contains only the mostsignificant elements of the original matrix A. Assuming that the s_(n-1)was very small and s_(n) was zero and we decide to eliminate thesecolumns from the original matrix, the result would be a (M)×(N-2) matrixmade from the first N-2 columns of U, S, & V, as shown in FIG. 3.

Note that Factor Analysis or any other kind of Principle ComponentAnalysis with dimensionality reduction might work just as well in thiscase.

Step 3: Phrase Clustering

The next step in creating a phrase thesaurus consists of clusteringphrases into classes based on the degree of overlap between distancevectors. A number of standard clustering algorithms have been describedin the literature. The most efficient ones include Single Link, CompleteLink, Group Average, and Ward's algorithm. These algorithms aretypically used to classify documents for information retrieval, and,depending on the particular data being modeled, one or the other hasbeen shown to be more efficient. For a discussion of clusteringalgorithms, see, e.g., (El Hamdouchi, A. and P. Willett, “HierarchicDocument Clustering using Ward's Method,” Proceedings of theOrganization of the 1986 ACM Conference on Research and Development inInformation Retrieval, 1988, pp. 149-156; El Hamdouchi, A. and P.Willett, “Comparison of Hierarchic Agglomerative Clustering Methods forDocument Retrieval,” The Computer Journal 32.3, 1989, pp. 220-227;Cutting, Douglas, R., David R. Krager, Jan O. Pedersen, John W. Tukey,“Scatter/Gather: A Cluster-Based Approach to Browsing Large DocumentCollections,” Proceedings of the 15^(th) Annual International SIGIR '92,Denmark, pp. 318-329.

All of these clustering algorithms are “agglomerative” in that theyiteratively group similar items, and “global” in that they consider allitems in every step.

We can use one or the other of these algorithms to cluster similarphrases into equivalence classes by performing the following steps:

-   a) Calculate all inter-phrase similarity coefficients. Assuming    q_(x) and q_(y) are any two phrases, they can be represented by rows    X & Y of A_(new) from Step 2, so the similarity between any two    phrases using the Cosine coefficient would be:

${s_{\cos}\left( {q_{x},q_{y}} \right)} = \frac{q_{x}^{T} \cdot q_{y}}{{q_{x}}\mspace{11mu}{q_{y}}}$

-   b) Assign each phrase to its own cluster-   c) Form a new cluster by combining the most similar pair of current    clusters (r, s)-   d) Update the inter-phrase similarity coefficients for all distances    using r & s.-   e) Go to step (c) if the total number of clusters is greater than    some specified number N.

Clustering algorithms differ in how they agglomerate clusters. SingleLink joins clusters whose members share maximum similarity. In the caseof Complete Link, clusters that are least similar are joined last, orrather an item is assigned to a cluster if it is more similar to themost dissimilar member of that cluster than to the most dissimilarmember of any other cluster. Group Average clusters items according totheir average similarity. Ward's method joins two clusters when thisjoining results in the least increase in the sum of distances from eachitem to the centroid of that cluster.

Clustering techniques tend to be resource intensive, and some initialseeding of clusters, based on rough guesses, may be necessary. TheBuckshot algorithm (Cutting, et. al., 1992) can be used to accomplishthis goal. Buckshot starts with a small random number of clusters andthen uses the resulting cluster centers (and just these centers) to findthe right clusters for the other items. One could imagine other similaralgorithms that take some initial guesses at the cluster center, andthen use the cluster center (or even the top N items that can beconsidered as the closest to the center), and find the other bucketsaccordingly.

We can use any one of these clustering algorithms or a combination ofthem depending on the computational resources required and other factorsto derive both flat and hierarchical groupings of phrases.

Step 4: Hand Tagging of Classes

In a final step, a sub-set of the hand-checked phrase classes are taggedwith abstract descriptors denoting abstract conceptual representationsof the phrases contained in each class. Descriptors include speech actclassifications for verb phrases (e.g. request [ . . . ], confirm [ . .. ], reject [ . . . ], clarify [ . . . ] etc. and object nouns (e.g.date, location, time, amount,) and proper names (businesses,restaurants, cities, etc.).

The phrases in a phrase thesaurus produced in accordance with theinvention can be arranged in a hierarchical manner. For example, phrasesthat can occur as part of other phrases can be represented once in thephrase thesaurus and each other phrase that can include such phrase caninclude a pointer to that phrase. This can be desirable to enable thephrase thesaurus to be represented more compactly, thus decreasing thedata storage capacity required to store the data representing the phrasethesaurus.

i. A Computer System for Automatically Creating Recognition Grammars

The phrase thesaurus described above can be implemented as part of acomputer system that can be used to automatically generate complexrecognition grammar for speech recognition systems. The recognitiongrammar can then be used with an interactive user interface that isresponsive to spoken input (voice input). The recognition grammarenables interpretation of the spoken input to the user interface. Thesystem combines call-flow design, network expansion, and grammarcompilation into a single development tool. The thesaurus forms the keyelement of this system, but in order to function in the manner desired,it must be integrated and work together with a number of other systemcomponents.

The system consists of the following components: (a) a graphical userinterface for designing and editing the call flow for a voiceapplication, (b) a network expander that retrieves alternative variantsfor the user commands specified in the call-flow design from thedatabase along with their probabilities, (c) a linguistic database, (d)an editor, and (e) a compiler that translates the grammar network into aformat than can be used by commercial speech recognizers.

(a) Call flow Design: The first step in designing a recognition networkfor a voice controlled dialogue system consists of specifying the callflow in such a way as to anticipate the logic of the interaction. Thesystem's graphical user interface allows for two different design modes:a graphical mode, and a text mode. In the graphical mode, the designerspecifies user requests, system states, and the transitions betweenthese states by using and manipulating icons that can be connected byarrows to indicate the logic of the interaction. The system comes with aset of standard icons (for greetings, yes/no, system confirmation,request for clarification, etc.), but the designer can define additionalicons. The user can subsequently add text to each node by clicking on anicon indicating a user request. FIG. 4 shows the initial part of a callflow for a simple restaurant in both graphical and text mode. For theuser request type: request restaurant information, the designer onlyneeds to specify one example, of making such a request. For each userrequest, the grammar specifies the set of legitimate variants. Note thatthe system will not recognize speech input that is not explicitlyspecified in the grammar.

(b) Network expander: In a second step, the user nodes in the call flowdesign are automatically expanded to include a near exhaustive set ofpossible user responses to any given system prompt. FIG. 5 shows thetype of network that needs to be generated to recognize the userresponse to the systems prompt “what kind of food you like to eat?” Foreach user request, the grammar specifies the set of legitimate variants.Note that the system will not recognize speech input that is notexplicitly specified in the grammar. If the recognition system allowsfor probabilistic grammars, the Network Expander can supply frequencyand other probabilistic bigram and trigram statistics to build such agrammar.

To activate the network expansion functionality, the designer clicks ona network expansion icon. This will prompt the system to take theutterances specified in each user node and automatically retrievealternative variants from the database. For example, suppose we want tomodel a user request for help. For the phrase “I need help,” the networkexpander will return: “What do I do now?,” “Help!,” “Help me, please,”“I could need some help here!,” “Can you help me?,” “I'm lost, I don'tknow what to do,” “Oops, something's wrong!,” etc.

Phrases can be expanded in the order of frequency of occurrence, thatis, most likely variants are listed first, with others following in theorder of descending frequencies. Expanded icons can be viewed as listsof phrases or in annotation mode. In this mode, the abstract meaningrepresentation(s) for the selected phrases can be accessed and modified.For phrases with overlapping or no representation, the designer cansupply customized representations required by the context.

(c) Linguistic Database: The linguistic knowledge required for theautomatic network expansion is stored in a large, machine-searchabledatabase. The database contains the phrase thesaurus (along withprobability scores associated with each phrase). In addition, itcontains lists of common nouns for filling phrase templates, as well aslocations, dates, proper names etc. The database is customizable, thatis, users can create their own application specific lists of objects,names etc.

(d) Editor: The grammar designer provides editing functionality at allstages in the design process. Initial call flow designs can be saved,retrieved, and changed in both graphical and text mode. After thenetwork has been expanded, the designer can go back to the initial callflow design, click on an icon, and view/edit the phrase variantsretrieved by the system. At this stage, most of the editing activitywill consist of eliminating variants that don't fit the pragmaticcontext, and of completing phrase templates by accessing thesupplemental databases provided by the system or by typing in thetemplate fillers directly. In the annotation mode, the user can reviewand modify the meaning representations automatically supplied by thesystem. At all times during the design process, users can view and edittheir work in any one of the modes provided by the system (graphicalcall-flow, call flow text mode, expansion mode).

(e) Compiler: After completing the editing, the user activates thesystem compiler, which executes a computer program that translates thegrammar network design into a format that can be used by the recognizer.

2. A Natural Language Understanding Component to be Used in SpeechRecognition Systems

In another aspect of the invention a compiled sub-set of the phrasethesaurus is incorporated into a speech recognition system to beaccessed at run-time in order to parse the incoming speech signal and toderive an abstract conceptual representation of its meaning that ispassed on to the application. The phrase subset used in the run-timenatural language interpreter is identical to the one used in aparticular grammar. (Recall that the grammar specifies the total set ofuser commands the system expects and is able to process. Commands notspecified in the grammar are automatically assigned to a single variablethat triggers a system request for clarification.)

To illustrate this aspect of the present invention, we describe how thegrammar and the natural language understanding component function withinthe context of a conventional speech recognition system. This aspect ofthe invention is particularly concerned with components 1 (e) and 2 inthe description provided below; the other components are part of atypical speech recognition system and are included in the description toclarify the operation of the invention.

The operation of a voice-interactive application entails processingacoustic, syntactic, semantic, and pragmatic information derived fromthe user input in such a way as to generate a desired response from theapplication. This process is controlled by the interaction of fourseparate but interrelated components, as shown in FIG. 6.

-   -   1. a speech recognition front-end consisting of (a) an acoustic        signal analyzer, (b) a decoder, (c) a recognition grammar, (d)        phone models, and (e) a phonetic dictionary    -   2. A Natural Language Understanding (NLU) component    -   3. a dialogue manager    -   4. a speech output back end (an application interface and        response generation component)

-   1. When a speech signal is received through a microphone or    telephone hand-set, its acoustic features are analyzed by the    acoustic signal decoder and a set n of the most probable word    hypotheses are computed based on the acoustic information contained    in the signal, and the phonetic transcriptions contained in the    dictionary. The recognition hypotheses are constrained by a    recognition grammar that defines the user choices and tells the    system what commands to expect at each point in a given interaction.    Because the grammar specifies only legitimate word sequences, it    narrows down the hypotheses generated by the acoustic signal    analyzer to a limited number of possible commands that are processed    by the system at any given point.

-   2. The NLU component translates the utterances specified in the    recognition grammar into a formalized set of instructions that can    be processed by the application.

-   3. The dialogue manager passes the commands received from the NLU    component on to the application via the application interface    (component 3) and processes the system response. This response can    be an action performed by the system, e.g., to access a database and    retrieve a piece of information. It can also be a verbal system    response, e.g. a request for clarification, “Do you want Edgar Smith    or Frank Smith?, or it can be a combination of both.

The speech-output back-end (component 4) takes the verbal responsegenerated by the dialogue manager and maps it to an acoustic speechsignal, using either a speech synthesizer or prerecorded utterances froma database. (For a comprehensive overview of state-of-the-art dialoguesystems, their operation, and assessment, see Ronald Cole, A. J.Mariani, Hans Uszkoreit, Annie Zaenen, Victor Zue, “Survey of the Stateof the Art in Human Language Technology,” Center for Spoken LanguageUnderstanding, Oregon Graduate Institute, 1995, and EAGLES, Handbook ofStandards and Resources for Spoken Dialogue Systems, De Gruyter, Berlin& New York, 1997.

This aspect of the invention particularly concerns the NLU component. Inconventional spoken dialogue systems, recognition grammars are mappedonto a set of formalized instructions by using a crude technique called“word spotting.” Word spotting proceeds from a given set of instructionsand then searches the user input for specific words that match theseinstructions. Word spotting works by disregarding utterances or parts ofutterances that are deemed irrelevant at a given state of theuser-machine interaction. Word spotting works for very simple systems,but it is limited by the fact that it cannot recognize negations or morecomplex syntactic relationships.

In the present invention, recognition grammars are mapped to systeminstructions by way of an annotation scheme that extracts the abstractmeaning from a number of alternative phrase variants. This is possiblebecause the underlying thesaurus database classifies phrases accordingto semantic similarity and contains pre-tagged descriptors for eachclass. At run-time, user speech input is parsed automatically intophrase-based units, which are subsequently translated into systeminstructions.

Various embodiments of the invention have been described. Thedescriptions are intended to be illustrative, not limitative. Thus, itwill be apparent to one skilled in the art that certain modificationsmay be made to the invention as described herein without departing fromthe scope of the claims set out below.

We claim:
 1. A phrase analysis system comprising: a phrase databaseconfigured to store phrases; and a phrase analysis computer coupled withthe phrase database and configured to: obtain a corpus comprising aplurality of phrases; extract phrases from the corpus; determine adegree of similarity among the phrases based on phrase contextsassociated with the phrases and on overlaps among phrase contextsdetermined by surrounding phrases in the corpus proximate to thephrases; and cluster the phrases into classes within the phrase databasebased on the degree of similarity among the phrase.
 2. The system ofclaim 1, wherein the corpus comprises a text corpus.
 3. The system ofclaim 1, wherein the corpus comprises a speech corpus.
 4. The system ofclaim 1, wherein the corpus comprises a conversational corpus.
 5. Thesystem of claim 1, wherein the corpus comprises a machine-readablecorpus.
 6. The system of claim 1, wherein the corpus comprises aspecialized corpus that is specific to a domain.
 7. The system of claim6, wherein the domain comprises an interactive discourse.
 8. The systemof claim 7, wherein the interactive discourse comprises at least one ofthe following: a task oriented interactive discourse, a verbaldiscourse, a simulated discourse, and a scripted discourse.
 9. Thesystem of claim 1, wherein the classes comprises equivalence classes.10. The system of claim 1, wherein the classes comprises tags denotingabstract conceptual representations.
 11. The system of claim 10, whereinthe classes comprises hand-checked tags.
 12. The system of claim 10,wherein the tags includes at least one of the following: a request, aconfirmation, a rejection, a clarification, an object name, a location,and a proper name.
 13. The system of claim 1, further comprising adialog manager configured to generate a response to a user based on auser-based natural language input from the user, the response generatedfrom phrases obtained from the phrase database.
 14. The system of claim13, wherein the user-based natural language input is obtained via aphone.
 15. The system of claim 1, where the phrases comprises normalizedphrases.
 16. The system of claim 1, where the corpus reflectinteractions in multiple languages.
 17. The system of claim 1, whereinthe phrases relate to a genre.
 18. The system of claim 1, wherein thephrases relate to food.
 19. The system of claim 1, wherein the phrasesrelate to a request for help.